1. Field of the Invention
The invention relates to a controller area network (CAN) communication system and method by which communications are carried out among a plurality of communication devices connected through a CAN bus and, more particularly, to a fault location device, communication device, and fault location method that are able to identify a fault point due to a one-wire break.
2. Description of the Related Art
A known CAN communication system carries out data communications among a plurality of communication devices. The plurality of communication devices are connected with one another through a CAN bus, and each have a CAN controller unit. The CAN communication protocol allows bidirectional serial communication through a differential serial bus.
In the CAN communication system, each communication device transmits data with its own identification (ID) code to the CAN bus. At the time the communication device transmits the data, when the CAN bus is not occupied by data transmitted from another communication device, the data transmitted from the communication device flows through the CAN bus and reaches an intended communication device. On the other hand, when the CAN bus is occupied by data from another communication device, the data, which will be transmitted from the communication device, is queued in the CAN controller unit. Then, when there is only one communication device in which the data are queued, the queued data flow through the CAN bus when the CAN bus is unoccupied. On the other hand, when there are a plurality of communication devices in which the data are queued, the data of the highest-priority communication device on the basis of the ID code from among those communication devices flow through the CAN bus prior to the other queued data.
The CAN communication system includes the plurality of communication devices (nodes). These plurality of communication devices are connected with one another through the CAN bus which is formed of twisted pair wires. The communication devices transmit and receive data to and from one another. The twisted pair wires of the CAN bus are bus wires, one of which is called CAN High (CANH) and the other one of which is called CAN Low (CANL). The communication devices may be, for example, formed of a plurality of electronic control units (ECUs) that control various portions of a vehicle.
Japanese Patent Application Publication No. 2003-143164 (JP-A-2003-143164), for example, describes a controller that is able to identify another faulty controller (ECU) at the time of intercommunication control. This controller observes a device that transmits and receives data to and from a communication line. The device includes a measurement unit that measures a period of time from the time when the preceding communication data are acquired to the time when the subsequent communication data are acquired, and a determination unit that determines a breakdown of communications when the measured time exceeds a predetermined determination time. When communication data, transmitted from another communication device to the communication line at a minimal communication cycle, are not acquired even when a transmission interval of the communication data exceeds a predetermined determination threshold, it is determined that a breakdown of communications occurs in that transmitting communication device and the device is faulty.
However, the above described related art has the following problems.
In the CAN communication system, when a one-wire break occurs, it is difficult to identify a communication device connected to a communication line in which the one-wire break is occurring. The one-wire break includes a break that occurs in one of communication lines connected to a communication device and a contact failure that occurs in one of the communication lines connected to a communication device. When the one-wire break occurs, it is no problem when the one-wire break continues and then can be repaired; however, there is a possibility that, for example, in the case of a contact failure, the communication line may be apparently connected before the one-wire break is repaired. In this case, the communication line is just apparently connected, so a contact failure occurs again when the communication line is pulled. Thus, a fault point may be identified by pulling the communication line. However, because a vehicle is equipped with several hundreds of communication lines, it is practically impossible to identify the communication device, in which the contact failure is occurring, by pulling the communication lines.
In addition, when a one-side communication line is broken, a communication device connected to the broken communication line generates an abnormal voltage. This corrupts data transmitted from another communication device. As a result, data (message) from a normal communication device are also corrupted. As a consequence of corruption of the message transmitted from the normal communication device, the normal communicable communication device is also determined to be in a breakdown of communications. Thus, the normal communicable communication device is erroneously determined to be faulty.
In the above described JP-A-2003-143164, it is not assumed that one of wires between CANH and CANL is broken. Thus, it is impossible to identify a communication device connected to a wire in a one-wire break.